South San Francisco-based Genentech, a unit of Roche, said today it has won FDA approval of obinutuzumab (Gazyva), as a new treatment for people with chronic lymphocytic leukemia, in combination with chlorambucil chemotherapy. The new Genentech drug (pronounced guh-ZY-vuh) was previously known as GA101, or as the next-generation follow-up molecule to one of the company’s greatest hits—rituximab (Rituxan). About 5,000 people die each year in the U.S. of chronic lymphocytic leukemia, making it one of the more common forms of blood cancer.

FDA approval of this medicine was about as close to a no-brainer as you see in drug development. That’s partly because Genentech had the guts to test its new treatment head-to-head against its own blockbuster, and show the new drug is superior. The latest approval is the fifth new Genentech cancer medicine approved by the FDA in the past three years, a time when many industry observers fretted it might lose its mojo under Roche ownership.

The new drug, in tandem with chlorambucil chemotherapy, was able to keep tumors from spreading for a median time of 23 months for patients with chronic lymphocytic leukemia, according to clinical trial results presented in June at the American Society of Clinical Oncology (ASCO). Patients who were randomly assigned to rituximab and the same chemo drug were able to keep their tumors in check for 15.7 months, while patients stayed that way for only 10.9 months on the chemo alone, according to results presented at ASCO. In today’s press release, Genentech offered some more data, including a line about how more than one-fourth of patients on the new drug and the chemo had their cancer completely wiped out (27.8 percent), compared with 0.9 percent who did that well on the chemo alone.

For biostatistics geeks out there, the new drug had an amazingly good hazard ratio of 0.16. That means the new drug and a chemo agent offered an 84 percent lower risk of disease worsening or death during the study than the chemo alone. A 20-30 percent reduced risk of disease worsening or death is more commonly seen among new cancer drugs. More detailed follow-up results are expected to be released at the American Society of Hematology conference next month in New Orleans.

Like all cancer drugs, the new product has side effects that aren’t trivial. About two-thirds of patients (67 percent) on the new drug combo reported moderate to severe side effects, most of which were infusion reactions the first time patients got the new treatment, according to data presented at ASCO. Patients on the new drug also reported a higher rate of neutropenia, a depletion of infection-fighting white blood cells, than was seen in the control group. About 34 percent of those on the new drug reported moderate to severe neutropenia, compared with 25 percent who got rituximab. About 16 percent of patients on the chemo alone had that moderate-to-severe form of neutropenia. Genentech said the increased rate of neutropenia didn’t appear to increase the rate of infections in patients on the new medicine.

Taken together, those results helped move Genentech’s application up the priority list at the FDA. The agency gave it “Breakthrough” status, and approved the medicine several weeks before its mid-December legal deadline to complete its regulatory review.

“Today’s approval represents an important new addition to the treatments for patients with CLL,” said Richard Pazdur, director of the FDA’s cancer drug office, in a statement. “This approval reflects the promise of the Breakthrough Therapy Designation program, allowing us to work collaboratively with companies to expedite the development, review and availability of important new drugs.”

The new drug is estimated to cost $41,300 for a course of treatment for chronic lymphocytic leukemia, said Genentech spokeswoman Emmy Wang. A course of treatment with the drug consists of four intravenous infusions in the first month, then once-monthly infusions from months 2-6, Wang says.

The new antibody came to Genentech through an unusual path, which I described in a feature back in June. Roche acquired GlycArt Biotechnology in 2005, and then Roche acquired all of Genentech in 2009. Genentech has long worked on various iterations on rituximab, particularly through a long-running partnership with Weston, MA-based Biogen Idec (NASDAQ: BIIB). Under that partnership, Biogen Idec provides 35 percent of the R&D costs of obinutuzumab, and stands to receive 35-39 percent of the U.S. sales of the drug based on hitting certain sales milestones, Wang said.

Although this is just the first approval in a specific patient population for obinutuzumab, the long-term potential of the drug is obviously huge. Rituximab, first approved for non-Hodgkin’s lymphoma by the FDA in 1997, generated $7.29 billion in worldwide sales last year, making it the world’s fifth-biggest selling drug, according to Genetic Engineering and Biotechnology News. The new product will need to prove its worth in multiple other patient populations to achieve what the original antibody has over time, but it’s not inconceivable.

The new drug is not just a simple knock-off, although it is designed to hit the same molecular marker—CD20. The key difference is that the new drug was designed through “glycoengineering” to remove a sugar group that would otherwise be attached to the backbone of the Y-shaped protein drug. That way, Genentech’s antibody engineers were hoping to improve upon a phenomenon they observed with rituximab known as “antibody-dependent cell-mediated cytotoxicity,” or ADCC. While rituximab was made to bind specifically with the CD20 target commonly found on the surface of tumor cells, its effectiveness couldn’t be completely explained by the antibody’s direct tumor-killing ability. The binding of the antibody to the target would also send a signal that alerted the immune system to start attacking those tumor cells. For the next-generation antibody, the goal was to hit the same CD20 target, but do a better job at triggering the critical immune response.